From the prior art, various absolutely operating optical angle transmitters are known, which are based on the transparent or reflecting scanning of a code disc with digital tracks (see for example DE 198 55 064 and DE 199 44 005). Such rotation transmitters require log 2(N) code tracks for N angle positions which are to be represented. With limited structural space, the aperture width for each light sensor therefore decreases by 1/log 2(N). At the same time, the aperture height decreases with 1/N. The aperture area (and hence the amount of light available for each light sensor) therefore decreases with 1/(N*log 2(N)), which greatly limits the accuracy able to be technically achieved.
To secure such rotation transmitters against any defects which may occur, such as for example a damaged code disc, further steps become necessary. One approach consists in a dual configuration of the required tracks for security. These steps, however, additionally intensify the light quantity problem. The purely digital sensors have, in addition, the disadvantage that the application of the codes has to be carried out equally accurately for all tracks.
Absolute angle transmitters are also known which are based on a purely analog scanning of a code disc (see for example DE 101 43 662). These angle transmitters have the disadvantage, however, that the analog track must be produced very accurately over a relatively large distance. The range of modulation of such transmitters is limited, and the required accuracy of the analog-to-digital converters connected downstream is relatively high. The solution approach of calibrating the transmitter once by means of a sufficiently great number of points along a relatively inaccurately produced track, and then interpolating in running operation along the calibration values which were found, fails in practice due to the ageing of the components involved.
From DE 196 04 502 a rotational transmitter is known in which a series of Gray code tracks is used and which achieves a refined angle accuracy by means of a sawtooth-shaped analog track. This arrangement has the disadvantage that at the points of change of the sawtooth, it produces the same brightness as in the centre of a tooth. Thereby, an ambiguity of the angle which is to be read out exists. In addition, it is not reliably protected against damage to the code tracks.
From DE 40 14 479 an angle transmitter is known which uses a series of binary code tracks and achieves a refined angle accuracy by means of an analog track. This arrangement has the substantial disadvantage that it is not reliably protected against damage to the code tracks. Furthermore, the analog track used, which is limited in a sinusoidal shape in the core, is not optimally formed for the improvement of the angle accuracy, because in particular in the immediate environment of the maximum or minimum of the light intensity, the gradient is almost zero.
From DE 100 06 675 a path or angle measurement arrangement is known, which has several Gray-coded digital tracks and an analog track rising or respectively falling in width continuously synchronously therewith. The scanning takes place by means of a regular arrangement of a plurality of light sensors, the spacing of which from each other is distinctly smaller than the analog track width. This arrangement has the disadvantage that the resolution improvement which is able to be achieved is limited by the number of light sensors allotted to the analog track width. With a relatively high required improvement to the accuracy, this technology only becomes attractive with a clear drop in price of the required light sensor array. Alternatively, the width of the analog track would have to be increased, which can lead to illumination problems and, in addition, burdens the structural space. A further disadvantage of the described arrangement is that with maximum utilization of the angle interval (between two digital code jumps) which is available for resolution improvement, to avoid ambiguities of the scanning values it requires a second track of the same kind and phase-displaced.
It is the object of the invention to provide a rotation transmitter and a method of scanning a code disc of a rotation transmitter which, using as few code tracks as possible, can achieve as high an accuracy as possible and in which the requirements for production accuracy are reduced as far as possible.